The present invention relates to photographic printing systems. In particular, the present invention is an improved photographic printer which automatically derives slope compensation factors for use in printing.
Photographic printers produce color or black and white prints or transparencies from photographic film originals (generally negatives). High intensity light is passed through the film and imaged on the photosensitive print medium (film or paper). The photographic emulsion layers on the print paper or film are exposed and subsequently processed to produce a print of the scene contained in the original.
A critical portion of a photographic printer is the exposure control, which controls the exposure of the photosensitive medium in order to ensure that the image on the photosensitive medium is properly exposed. The exposure control may utilize inputs from several different sources in order to determine the proper exposure. For example, most automatic printers use large area transmission density (LATD) sensors to sample light transmitted by the negative either prior to or during the exposure. Control of the exposure is determined using a method known as "integration to grey". In addition, many automatic printers include an automatic density correction (ADC) or color scanning station which scans the negative prior to printing and corrects the exposure in the event of a condition known as "subject failure". Finally, the operator may enter density correction signals from the operator control panel. Based upon some or all of these input signals, the exposure control determines the proper exposure for each of the color channels or for one black and white channel.
Photographic printers often include a "slope" adjustment which modifies exposures. These "slope" corrections have been used to correct for a number of deviations from normal printing conditions.
Originally, slope corrections were used to compensate for paper reciprocity. Although exposure of a photographic medium (which equals the product of light intensity and exposure time) may be maintained constant, the resulting density varies as a function of exposure time because of paper reciprocity. For example, if the exposure time is doubled and the intensity of the light is halved, the exposure is the same but the density is typically not the same due to paper reciprocity.
Later, slope was used to compensate for other factors. For example, it was found that certain negatives or certain scenes were not properly printed if the "integration to grey" method was strictly followed. For example, if the negative is dense in comparison to a normal negative, it may be desirable for the print to be of a different density than would be achieved by integration to grey. Similarly, a negative which is less dense than normal may also print best if the resulting print is of a different density than neutral grey. Slope has been used to compensate for these deviations from normal printing.
Still another factor which slope has been used to compensate for is non-linearity in the negative material. Slope, which is sometimes called over- or under-correction, has also been used to compensate for other factors which would otherwise lead to incorrect or non-optimal printing.
Due to the complexity of the slope correction and the multiple uses to which the correction can be put, determination of the proper slope correction is often difficult. In the prior art, the photographic printers have required the user of the printer to determine the desired slope values by trial and error and supply them to the printer as numerical values or as control settings. Determination of the proper slope values, therefore, can be a difficult and time-consuming task for the user.